Bones may be reinforced with rigid structures, for example plates, having apertures through which bone screws, for example polyaxial screws, may pass. Such reinforcing is useful for addressing fractures or other trauma, correcting alignment, or treating disease. A bone plate is used to span a fracture, or to join different bones. It is important that a bone screw securing a bone plate does not loosen and separate from the plate postoperatively. This separation or backing out may be a result of the screws failure to achieve a sufficient purchase in the bone, although the stripping of the screws has also been known to cause this problem, or can be a result of the forces placed upon the screws from spinal movement. Regardless of the cause of the hardware failures, a surgeon must repair or replace the broken or separated parts, requiring an undesirable invasive procedure.
Devices for preventing this separation may be associated with the plate or screw. Examples include a cover positioned over all or part of the screw head, or an expansion head screw.